In the manufacture of switch assemblies, such as used in multifrequency telephone pushbutton dial assemblies, a plurality of flat contact springs are mounted near one end between two mating plastic details so that a contact end of each spring projects from a mounting section secured between the plastic details. In the past, it has been customary to mount the springs at predetermined locations within a cavity formed in a plastic mounting frame, and then to weld a thermoplastic locking insert to the frame so as to fasten the mounting section of each spring between the two mounting details. The locking insert is formed with a flat top portion and a depending finger portion that fits closely within the cavity so that the spring section fits closely between an outer wall of the finger and an inner wall of the cavity.
The insert is formed with V-shaped "energy directors" (pointed teeth) along portions of the under surface thereof, which fit against flat surface areas of the frame. After the insert is preassembled with the frame, ultrasonic energy is applied to the members while squeezing the parts together, so that the plastic melts, starting at the tips of the energy directors, and the melted plastic surface bonds the two plastic details together, similarly to seam selding, at the regions where the energy directors are placed.
In this prior art method, the contact springs are secured in place only by using close fitting plastic parts and, if the finger member is too small with respect to the cavity, the spring can move longitudinally or vertically in some cases; if too large, the parts either cannot be assembled, or where they can, the wall of the cavity is deformed as pressure is applied, which operates to misalign the contact end of the spring.